1. Field of the Invention
This invention relates to the design of cylindrical slot-array antennas used for broadcasting in the VHF and UHF bands.
2. Background
Television broadcasters are concerned with providing sufficient transmission signal to the maximum number of customers within the constraint of the permissible transmitted power. It is desirable to match the broadcast antenna design as closely as possible to the distribution of customers, the terrain, and the propagation conditions around the transmitter location. For the digital TV market in the United States, for example, the aim is to control the radiation pattern to maximize the number of viewers reached by the FCC regulated transmission license for each area. New requirements call for antennas in which not only the gain but also the beam tilt can be adjusted for each azimuth.
Television signals are commonly broadcasted from slot-array antennas. A diagram of a typical cylindrical slot-array antenna 10 is shown in FIG. 1, together with a coordinate system for analyzing the antenna pattern. The antenna comprises an array of radiating slots 12 cut into a circular cylindrical waveguide 14 that stands vertically at the broadcast site. The waveguide is a metal pipe supporting an interior guided wave to carry energy to the slots. To support a single guided wave, the waveguide commonly has a metal inner conductor 16 along its axis and is therefore a coaxial waveguide. The input to the antenna is at one end, where the guided wave passes through a matching network to ensure that the antenna will operate across the required broadcast band. At the other end is a termination that is usually a short circuit. The slots may have a circumferential orientation to generate vertically polarized radiation or an axial orientation to generate horizontally polarized radiation. They may also be tilted at an arbitrary angle, and different slots may have different tilts. Mounted at or near each slot there may be a metal probe that protrudes into the waveguide, coupling energy from the guided wave to radiate through the slot.
The radiation pattern of a broadcast antenna is the variation of radiated power, of either vertical or horizontal polarization, as a function of azimuth and elevation angles. At any azimuth angle, the elevation pattern is the variation of radiated power as a function of elevation angle at a constant distance from a chosen origin. The radiated power in any direction may be expressed as a gain with respect to a reference antenna. Instead of azimuth and elevation the coordinates xcex8 and xcfx86 shown in FIG. 1 may be used according to the relationships
azimuth=360xc2x0xe2x88x92xcfx86
elevation=90xc2x0xe2x88x92xcex8
where xcex8 and xcfx86 are expressed in degrees. Broadcast antennas are designed so that the peak of the elevation pattern is slightly below the horizon, directing the strongest possible signal to distant customers. The elevation angle of the peak is called the beam tilt.
The positions of slots on the cylinder may be specified by their (xcfx86,z) coordinates where z increases from the input to the termination. A group of slots with the same z coordinate is called a bay. Present antenna designs have slots arranged in a (xcfx86,z) grid, and therefore comprise a set of identical bays spaced in z along the cylinder. The elevation patterns of such an antenna all have the same shape, and therefore the same beam tilt, regardless of azimuth. The variation of the peak gain with azimuth is determined by the arrangement of slots within a bay. The peak gain may be nearly constant or strongly varying, according to the requirements of the broadcast location.
The present invention uses the arrangement of slots in a cylindrical slot-array antenna to control the radiation pattern. In particular, an irregular arrangement permits the gain and/or beam tilt to vary with azimuth. Alternatively, an irregular variation of some property of the slots or their probes may be used to achieve a varying gain and beam tilt, even when the slots are positioned in a regular manner. There are many possible design variables. Slots may or may not have different dimensions. Slots may be axial, circumferential, tilted at an arbitrary angle, or a combination of these. Each slot may or may not have a coupling probe in its aperture or protruding into the waveguide or outside the cylinder. A coupling probe is used to excite a radiating field in the aperture of a slot that would not normally radiate; for example, axial slots in a coaxial waveguide require probes.
The present invention also provides an antenna design methodology that achieves efficient and rapid optimization by minimizing the number of degrees of freedom and the number of significant mutual-coupling interactions. A useful range of designs is achieved by requiring that the slots are arranged in bays and that all slots, and their probes, are identical. Designs of this kind are more convenient to manufacture and have greater structural integrity than designs with arbitrary slot placement or different slot and probe dimensions. Because, to achieve beams near the horizon, bays are separated by approximately a wavelength, mutual coupling between bays can be ignored, thereby speeding the design process significantly. Grouping slots into bays also assists in achieving a tunable designxe2x80x94that is, an antenna that will operate across the required broadcast band.
Although the antenna slots are physically grouped into bays, the analytical approach groups the slots into slot paths defined by a simultaneous variation in z and xcfx86 coordinates of the slots. Constant increments in both z and xcfx86 produce a helix that twists around and along the cylinder; in existing designs xcfx86 is constant, producing a vertical column. Bay alignment is maintained by having identical z variations in all slot paths. The variations of all array parameters, such as slot position (xcfx86,z), slot size and tilt, probe dimensions, and equivalent circuit parameters, are specified using the method of increments described below.
Grouping slots into paths is important because changing xcfx86 variations allows a useful control of the radiation pattern with a very small number of free parameters. This leads to an efficient optimization. For example, an array with 4 slots in each bay is represented by 4 slot paths. Bay alignment requires each slot path to have the same z variation and tuning the antenna typically requires 3 degrees of freedom for z in the method of increments. Useful pattern shaping can be done with 2 degrees of freedom for the xcfx86 variation in each slot path, in which case the whole array geometry is specified using just 3+4xc3x972=11 parameters.
Arranging slots in bays reduces drastically the number of coupling interactions that need to be considered. Coupling interactions in a bay are accounted for by perturbing the shunt admittances that represent slotsxe2x80x94replacing element admittances by active admittances. The analysis also justifies using the same equivalent circuit for slots and bays.
By making all of the slots identical, the mutual admittance between each pair of slots in a bay can be interpolated from a few pre-calculated values. The scattering matrix for coupling interactions can then be calculated and the admittance perturbations depend simply on the scattering matrix elements. This procedure allows optimization to include coupling effects with almost no speed penalty.